The individual cannot know an objective reality, but only what one perceives. Common sense would hold that that which actually knows (objects) is not itself a sense organ, but is the mind or consciousness. There is no way that humans can perceive actual objects or matter. One can only perceive a sense, how something feels to one and not what is actually "out there." Thus in the definition of this sense being a feeling, one would have to realize that it is therefore something that is mental. An example of this is the size of a cheese might's leg: to humans the leg of a cheese might appears so small that one has to use a microscope to view it, but on the other hand, to a cheese might the leg appears to be a lot larger and in proportion to its body. There is no true way to measure the length of the leg because it is all subjective and relative to the object used to measure to the viewer. Size then is relative to the view and is not an immutable intrinsic property.
Another example to express this relativity is that of a steeple: the information we have is circular since it is all relative and dependent on each other. For instance, standing at different distances from the steeple one perceives the steeple to have varying heights. Therefore it is not possible to distinguish which height is the "true" height. Even when one uses math and a measuring stick to find out how many feet tall it is that measurement is relative to the measuring stick. Also, when one tries to figure out how tall, say, 150ft is, they have to look at the steeple or something like that to be able to comprehend that mathematical expression.
Therefore, there is no reasonable, objective way to know what reality is. All of our perceptions are relative to us. Anais Nin said something along the same lines: "We don't see things as they are, we see them as we are." Because of this inability to know objective reality, we are forced to know only what is within us. We are forced to live basically in a solipsistic world and reality. Because of this, there is no way another person can believe that his or her beliefs are in any way better than another's or is more right. It must be accepted, because of our "limitations," that the only truths one can know are personal truths. This does not mean there could and might be some overlying Absolute Truth or Truths; only that there is no way we can, as humans, really reach it universally. I think that all the truths people find within themselves are that Absolute Truth, but because we can only view everything subjectively, they all appear to be different through interpretation. Also, I think that just because everything is subjective is in no way a legit excuse to go on mass-murdering rampages. Despite the fact that I cannot truly know that there are other people in the world, that if I decided to kill hordes of people they would be real people dying and not figments of my imagination, it is still wrong to do this. Maybe it is just something morally ingrained within me, but I think that either way it is a negative and unhealthy, wrong thing to do. Because even if it is all me, I would then be killing part of myself. It is like what the old Chinese sage, Chuang-tzu said: "Once I dreamed I was a butterfly, and now I no longer know whether I am Chuang-tzu, who dreamed I was a butterfly, or whether I am a butterfly dreaming that I am Chuang-tzu." You cannot know whether other people are a part of you, you a part of them, or separate entities unto themselves… but it does not matter. Either way you hurt other people like yourself or a hidden unaware part of yourself.
Other links:
you have to accept reality
what is really real?
if we define things as unreal, they may still be real in their consequences
**the theory of reality
Broad concepts:
subjectivism
relativism
epistemology
solpisism
philosophy
berkely
circular logic